1. Field of the Invention
The invention relates to a method of generating a position-dependent correction waveform by generating p functions (f.sub.l (I1), . . . ,(f.sub.p (I1)) from position information (I1), in which p is a natural number larger than 1.
The invention also relates to a waveform-generating circuit comprising a function-generating circuit coupled to receive position information to supply p functions (f.sub.l (I1), . . . ,f.sub.p (I1)) of the position information, in which p is a natural number which is larger than 1.
The invention further relates to a picture display device in which the method is used for generating a position-dependent correction waveform.
Position-dependent correction waveforms are used in, inter alia dynamic convergence circuits, dynamic focusing circuits, east-west and north-south correction circuits, and luminance uniformity circuits.
2. Description of the Related Art
Such a waveform-generating circuit is known from GB-A-2010052. The known waveform-generating circuit generates convergence waveforms for driving convergence coils of a display tube.
A picture displayed on a display screen is formed in known manner in that an electron beam consisting of separate electron beams (generally representing the primary colors red, green and blue) is horizontally deflected (parallel to an x axis) by means of a horizontal deflection circuit comprising a horizontal deflection coil, and vertically deflected (parallel to an y axis) by means of a vertical deflection circuit comprising a vertical deflection coil. A convergence error, which may occur as a result of the fact that the separate electron beams reach the display screen with a mutual offset (in the x and/or y direction), is corrected by driving convergence coils with the convergence waveforms generated by the waveform-generating circuit. The extent of a required convergence correction depends on the design of, and tolerances in a combination of the display tube and the associated horizontal and vertical deflection coils. The convergence waveforms should have a shape in conformity with the convergence correction required by said combination. Moreover, the convergence waveforms should be adjustable so as to match the shape as much as possible with the required convergence correction and to compensate for said tolerances.
The known waveform-generating circuit receives a horizontal position signal, for example, the position on the x axis, and divides this horizontal position signal into p horizontal sub-areas with which the display screen may be considered to be divided into p vertical strips parallel to the y axis. The square of the horizontal position signal is generated in each horizontal sub-area and multiplied by an associated coefficient ai. Similarly, a vertical position signal, for example, the position on the y axis, is divided into q vertical sub-areas with which the display screen may be considered to be divided into q horizontal strips parallel to the x axis. The square of the vertical position signal is generated in each vertical sub-area and multiplied by an associated coefficient bj. The convergence waveform is obtained by adding all (p+q) coefficient-multiplied squares. In this case, the convergence correction can be adjusted to obtain an optimum convergence correction at p+q positions on the display screen by setting the coefficients associated with selected functions of the horizontal and vertical position information. The selected functions define the shape of the convergence waveform and are the squares in this case. The quality of the convergence correction between the adjustment positions is determined by the fact in how far the selected functions satisfactorily match the convergence correction required by the combination of the display tube and the associated horizontal and vertical deflection coils.
A drawback of the known waveform-generating circuit described in GB-A-2010052 is a small flexibility because exclusively squares are generated. The circuit is not very complex.
Such a waveform-generating circuit is also known from EP-A-0516229, corresponding to U.S. Pat. No. 5,435,536. An analog embodiment of the known waveform-generating circuit generates r powers of horizontal position information and s powers of vertical position information. Moreover, each r power of the horizontal position information is multiplied by each s power of the vertical position information for generating r times s crossterms.
A coefficient circuit supplies r coefficients, each of which is to be multiplied by a corresponding power of the horizontal position information, s coefficients, each of which is to be multiplied by a corresponding power of the vertical position information, and r times s coefficients, each of which is to be multiplied by a corresponding cross-term. The waveform generated by the known waveform-generating circuit is obtained by adding all r and s coefficient-multiplied powers to the r times s coefficient-multiplied cross-terms.
A digital embodiment of the known waveform-generating circuit converts the analog horizontal and vertical position information, into digital position words, whereafter a microcomputer computes the waveform. To this end, said powers and cross-terms are computed on the basis of the digital position words, and each of said powers and cross-terms is multiplied by an associated coefficient and added together to form a sum. The coefficients are stored in a non-volatile, writable memory. The sum is convened into the waveform via a D/A converter. The microcomputer is often not fast enough so that upon start-up of the waveform-generating circuit, a scratch memory is filled with a coefficient-based interpolation software which comprises a much larger number of points than the number of coefficients. This working memory is addressed with the digital position words.
In this case, the convergence correction can be adjusted to obtain an optimum convergence correction at r plus s plus r times s positions on the display screen by setting the coefficients associated with selected functions of the horizontal and vertical position information, in this case, powers of the horizontal and vertical position information and their crossterms.
A drawback of the waveform-generating circuit known from EP-A-0516229 is its great complexity: many powers and cross-terms with their associated coefficients are generated. However, this is accompanied by a great flexibility.